Apparatus for polishing thin, flat semi-conductor wafers is well-known in the art. Such apparatus normally includes a polishing head which carries a membrane for engaging and forcing a semiconductor wafer against a wetted polishing surface, such as a polishing pad. Either the pad, or the polishing head is rotated and oscillates the wafer over the polishing surface. The polishing head is forced downwardly onto the polishing surface by a pressurized air system or, similar arrangement. The downward force pressing the polishing head against the polishing surface can be adjusted as desired. The polishing head is typically mounted on an elongated pivoting carrier arm, which can move the pressure head between several operative positions. In one operative position, the carrier arm positions a wafer mounted on the pressure head in contact with the polishing pad. In order to remove the wafer from contact with the polishing surface, the carrier arm is first pivoted upwardly to lift the pressure head and wafer from the polishing surface. The carrier arm is then pivoted laterally to move the pressure head and wafer carried by the pressure head to an auxiliary wafer processing station. The auxiliary processing station may include, for example, a station for cleaning the wafer and/or polishing head; a wafer unload station; or, a wafer load station.
More recently, chemical-mechanical polishing (CMP) apparatus has been employed in combination with a pneumatically actuated polishing head. CMP apparatus is used primarily for polishing the front face or device side of a semiconductor wafer during the fabrication of semiconductor devices on the wafer. A wafer is "planarized" or smoothed one or more times during a fabrication process in order for the top surface of the wafer to be as flat as possible. A wafer is polished by being placed on a carrier and pressed face down onto a polishing pad covered with a slurry of colloidal silica or alumina in de-ionized water.
A schematic of a typical CMP apparatus is shown in FIGS. 1A and 1B. The apparatus 10 for chemical mechanical polishing consists of a rotating wafer holder 14 that holds the wafer 10, the appropriate slurry 24, and a polishing pad 12 which is normally mounted to a rotating table 26 by adhesive means. The polishing pad 12 is applied to the wafer surface 22 at a specific pressure. The chemical mechanical polishing method can be used to provide a planar surface on dielectric layers, on deep and shallow trenches that are filled with polysilicon or oxide, and on various metal films. CMP polishing results from a combination of chemical and mechanical effects. A possible mechanism for the CMP process involves the formation of a chemically altered layer at the surface of the material being polished. The layer is mechanically removed from the underlying bulk material. An altered layer is then regrown on the surface while the process is repeated again. For instance, in metal polishing a metal oxide may be formed and removed repeatedly.
A polishing pad is typically constructed in two layers overlying a platen with the resilient layer as the outer layer of the pad. The layers are typically made of polyurethane and may include a filler for controlling the dimensional stability of the layers. The polishing pad is usually several times the diameter of a wafer and the wafer is kept off-center on the pad to prevent polishing a non-planar surface onto the wafer. The wafer is also rotated to prevent polishing a taper into the wafer. Although the axis of rotation of the wafer and the axis of rotation of the pad are not collinear, the axes must be parallel. Polishing heads of the type described above used in the CMP process are shown in U.S. Pat. No. 4, 141180 to Gill, Jr., et al.; U.S. Pat. No. 5,205,082 to Shendon et al; and, U.S. Pat. No. 5,643,061 to Jackson, et al. It is known in the art that uniformity in wafer polishing is a function of pressure, velocity and the concentration of chemicals. Edge exclusion is caused, in part, by non-uniform pressure on a wafer. The problem is reduced somewhat through the use of a retaining ring which engages the polishing pad, as shown in the Shendon et al patent.
Referring now to FIG. 1C, wherein an improved CMP head, sometimes referred to as a Titan.RTM. head which differs from conventional CMP heads in two major respects is shown. First, the Titan.RTM. head employs a compliant wafer carrier and second, it utilizes a mechanical linkage (not shown) to constrain tilting of the head, thereby maintaining planarity relative to a polishing pad 12, which in turn allows the head to achieve more uniform flatness of the wafer during polishing. The wafer 10 has one entire face thereof engaged by a flexible membrane 16, which biases the opposite face of the wafer 10 into face-to-face engagement with the polishing pad 12. The polishing head and/or pad 12 are moved relative to each other, in a motion to effect polishing of the wafer 10. The polishing head includes an outer retaining ring 14 surrounding the membrane 16, which also engages the polishing pad 12 and functions to hold the head in a steady, desired position during the polishing process. As shown in FIG. 1C, both the retaining ring 14 and the membrane 16 are urged downwardly toward the polishing pad 12 by a linear force indicated by the numeral 18 which is effected through a pneumatic system.
In the improved CMP head 20 shown in FIG. 1C, large variations in the removal rate, or polishing rate, across the whole wafer area are frequently observed. A thickness variation across the wafer is therefore produced as a mean cause for wafer non-uniformity. The improved CMP head design, even though utilizing a pneumatic system to force a wafer surface onto a polishing pad, the pneumatic system cannot selectively apply different pressure at different locations on the surface of the wafer. For instance, as shown in FIG. 1D, a profilometer data obtained on an 8-inch wafer is shown. The thickness difference between the highest point on the wafer and the lowest point on the wafer is almost 2,000 .ANG. yielding a standard deviation of 472 .ANG., or 6.26%. The curve shown in FIG. 1D is plotted with the removal rates in the vertical axis and the distance from the center of the wafer in the horizontal axis. It is seen that the removal rates at the edges of the wafer are substantially higher than the removal rate at or near the center of the wafer. The thickness uniformity on the resulting wafer after the CMP process is therefore very poor.
The polishing pad 12 is a consumable item used in a semiconductor wafer fabrication process. For instance, under normal wafer fab conditions, the polishing pad must be replaced after a usage of between 12 and 18 hours. Polishing pads may be hard, incompressible pads or soft pads. For oxide polishing, hard, incompressible and thus stiffer pads are generally used to achieve planarity. Softer pads are frequently used to achieve improved uniformity and smooth surfaces. The hard pads and the soft pads may also be combined in an arrangement of stacked pads for customized applications.
A problem frequently encountered in using polishing pads in a CMP process for oxide planarization is the rapid deterioration in polishing rates of the oxide with successive wafers. The cause for the deterioration has been shown to be due to an effect known as "pad glazing" wherein the surface of the polishing pads become smooth such that the pads can no longer hold slurry in-between the fibers. This has been found to be a physical phenomenon on the surface, and is not caused by any chemical reactions between the pad and the slurry.
To remedy the pad glazing effect, numerous techniques of pad conditioning or scrubbing have been proposed to regenerate and restore the pad surface and thereby, restoring the polishing rates of the pad. The pad conditioning techniques include the use of silicon carbide particles, diamond emery paper, blade or knife for scrapping the polishing pad surface. The goal of the conditioning process is to remove polishing debris from the pad surface, reopen the pores, and thus forms micro scratches in the surface of the pad for improved life time of the pad surface. The pad conditioning process can be carried out either during a polishing process, i.e., known as concurrent conditioning, or after a polishing process.
While the pad conditioning process improves pad consistency and its lifetime, conventional apparatus of a conditioning disk is frequently not effective in conditioning a pad surface. For instance, a conventional conditioning disk for use in pad conditioning is shown in FIGS. 2A and 2B. The conditioning disk 30 is formed by embedding or encapsulating diamond particles 32 in nickel 34 coated on the surface 36 of a rigid substrate 38. FIG. 2A is a cross-sectional view of a new conditioning disk with all the diamond particles 32, 42 embedded in nickel 34. After repeated usage as a conditioning disk, the cross-sectional view of the disk 30 is shown in FIG. 2B which shows that diamond particle 42 has been lost and the top surfaces of the remaining particles 32 are flattened. The loss of diamond particle from nickel encapsulation 34 occurs frequently when the particle is not deeply embedded in the nickel metal 34. In the fabrication of the diamond particle conditioning disk 30, a nickel encapsulation 34 is first mixed with a diamond grit which included the diamond particles 32, 42 and applied to the rigid substrate 38. The bonding of the diamond particles 32, 42 is frequently insecure and thus the particles are easily lost from the nickel coating during usage. The diamond particle 42 which is lost from the nickel encapsulation 34 may be trapped between the surfaces of the polishing pad and the wafer and causes severe scratches on the wafer. Another drawback for the diamond conditioning disk is that the pad conditioning efficiency decreases through successive usage of the disk since the top surfaces of the diamond particles are flattened after repeated usage when the diamond grit mechanically abrades the pad surface.
FIG. 3 is a graph illustrating the dependence of the removal rate on the pad disk life for a polishing pad conditioned by a conventional diamond grit conditioning disk. Four different polishing heads were measured for their removal rates which are indicated as h1, h2, h3 and h4. The removal rate is expressed in the thickness of the oxide layer removed in units of .ANG. per minute, while the pad life is expressed in the number of wafers polished. It is seen from FIG. 3 that when a new conditioning disk is used, the removal rate of the polishing pad is at about 4100 .ANG./min. The removal rate gradually deteriorates after a large number of wafers are polished while being conditioned by a diamond grit conditioning disk. The removal rate deteriorates to as low as 2700 .ANG./min, at which time a new polishing pad is used which improves the removal rate to about 3800 .ANG./min. However, the improved removal rate quickly deteriorates to an almost constant level of about 3300 .ANG./min. This level of removal rate is kept even after a second polishing pad is replaced. The ineffectiveness of the conditioning disk with a diamond grit is therefore evident from data shown in FIG. 3. The repeated use of a conditioning disk formed of a diamond grit loses its effectiveness after successive usage on more than 300 wafers. The replacement of new polishing pads does not improve the removal rate when the same conditioning disk is used due to the pad glazing effect.
The mechanism for chemical mechanical polishing of metal is different and more complex than the polishing of silicon oxide. It is generally believed that during the CMP of metal, metal forms an oxide layer on the surface which is subsequently removed by the polishing pad by a mechanism similar to that for oxide polishing. For instance, a mechanism that involves hydroxylation, bond formation with slurry and then, bond breaking from wafer. After the metal oxide layer is removed from the metal surface, metal is etched by the chemicals in the slurry solution, while simultaneously the exposed metal forms a new passivation layer through oxidation by the slurry solution. In practice, it is believed that three separate processes of the removal of metal oxide, the metal etching and the metal passivation occur simultaneously. A polishing slurry solution for a metal CMP therefore contains different components of fine slurry particles, i.e., a corrosion or etchant agent and an oxidant. The eventual planarization of the metal surface is achieved by the rigidity and planarity of the polishing pad similar to a process of oxide polishing.
When the metal being polished in the CMP process is copper, the polishing process becomes more complicated due to the characteristics of copper. Since copper is frequently used in multi-level interconnect structures in semiconductor devices, i.e., in damascene or dual damascene structures, a CMP step for forming copper interconnects in the damascene structures which produces satisfactory polishing uniformity becomes an important link in the entire fabrication process.
In a co-pending application, Ser. No. 09/368,294 which was assigned to the common Assignee of the present application and which is incorporated hereby by reference in its entirety, the effects of surface chemistry of a copper conductor during a chemical mechanical polishing process are shown.
Referring now to FIG. 4, wherein a graph illustrating the surface chemistry of a cooper conductor during a chemical mechanical polishing process, specifically, the dependency of the formation of cuprous oxides on the acidity or alkalinity of the slurry solution is shown. For instance, when the slurry solution is most acidic, i.e., having a pH between about 1 and 2, no cuprous oxide is formed on the surface of the copper. When the acidity of the slurry solution is increased to approximately a pH of 3, a layer of Cu.sub.2 O is formed on the copper surface while a layer of CuO is formed on the Cu.sub.2 O layer. When the acidity of the slurry solution is further decreased to a pH value between about 4 and about 6, cubic structured Cu.sub.2 O is produced on the surface of the copper while CuO is produced on the surface of the cubic Cu.sub.2 O. At still lower acidity, at a pH value between about 7 and about 9, similar layers of Cu.sub.2 O and CuO are produced on the surface of the copper conductors.
The graph shown in FIG. 4 therefore supports the present invention novel method in that a basic-type slurry solution is more effective in forming oxidation layers of copper on the surface of the copper conductors and thus facilitates the removal of cuprous oxides from the surface and achieving the copper removal objective.
In recent years, the copper CMP process has become an important process step in the fabrication of multi-level interconnects by the damascene structure. One major obstacle associated with the formation of the copper damascene is the CMP planarization process. During CMP, the process stability from wafer-to-wafer plays an important role in producing reliable damascene structures. The process stability in turn is determined by the effectiveness of the conditioning pad in conditioning the polishing pads.
It is therefore an object of the present invention to provide an apparatus for chemical mechanical polishing a semiconductor wafer that does not have the drawbacks or shortcomings of the conventional CMP apparatus.
It is another object of the present invention to provide an apparatus for chemical mechanical polishing a semiconductor wafer that has extended polishing pad life.
It is a further object of the present invention to provide an apparatus for chemical mechanical polishing a semiconductor wafer with extended pad life which includes a cleaning solution dispenser mounted on the conditioner arm such that a cleaning solution may be delivered to the conditioning pad.
It is another further object of the present invention to provide an apparatus for chemical mechanical polishing a semiconductor wafer with extended pad life by utilizing a first spray nozzle for dispensing a slurry solution and a second spray nozzle mounted juxtaposed to a conditioning pad for dispensing a cleaning solution capable of dissolving polishing debris on the polishing pad surface.
It is still another object of the present invention to provide an apparatus for chemical mechanical polishing a semiconductor wafer with extended pad life by utilizing a cleaning solution dispenser on the conditioning pad such that the pad glazing problem on the polishing pad can be substantially eliminated.
It is yet another object of the present invention to provide a method for chemical mechanical polishing a semiconductor wafer that has extended polishing pad life by utilizing a two-step cleaning process for the polishing pad.
It is still another further object of the present invention to provide a method for chemical mechanical polishing copper on a semiconductor wafer with extended pad life by a two-step cleaning method for dissolving oxides of copper formed on the polishing pad.
It is yet another further object of the present invention to provide a method for chemical mechanical polishing copper conductors on a semiconductor wafer with extended pad life by utilizing a first cleaning solution to dissolve polishing debris of copper oxides on the pad surface and then a second cleaning solution for removing the dissolved debris from the pad surface.